Less than 48 hours before a deranged Long Island woman killed her three kids, a child-services worker was called to the home because the drug-addled mom had threatened to harm the children, sources said yesterday.

But nobody was home both times a worker knocked on the door Friday at 819 Prospect Ave. in New Cassel, the sources said. It is unclear if a third attempt was made Saturday – a day before the kids were found dead.

Nassau County’s Child Protective Services had previously checked on the family as many as nine times in the past five years – without ever removing the children, the sources said.

“They had the key to put her away, they had the key to save those kids, they had the key to give me my kids, and they just kept giving her more chances,” the dad said. “They would not listen to anything I said.”

Demesyeux called Child Protective Services on Friday to say Brewer had been using drugs and told him she would harm the kids, the sources said.

Brewer’s grandmother, Maebelle Mickens, said she called the agency six times since 2006 to report Brewer beat the kids, left them alone and kept them from school.

Authorities didn’t take Brewer’s kids away, and a child-welfare worker “told me I should be ashamed of myself,” Mickens claimed.

County police believe Brewer, 27, drowned the kids and laid their corpses on her bed before leaping out a second-story window, fracturing a vertebrae.

Brewer dialed 911 and “said she killed her babies, even spelled her name for the 911 operator,” said Detective Lt. Michael Fleming. “The children looked . . . like they were cuddled up together for the night.”

Fleming said Jewell’s throat was cut and that cops recovered bleach that might have been used to first poison the kids.

Demesyeux yesterday viewed his sons’ bodies at the Medical Examiner’s Office, where he was sadly struck by how much bigger Innocent Jr. had gotten since he last saw the child.

The killings came a day before Brewer, who has a lengthy arrest record, was due to appear at a Family Court hearing, where Demesyeux believed he would be granted custody of his two sons.

“She knew I was gonna get them today,” he said.

He said in his call to Child Protective Services on Friday that he also said Brewer told him people on TV were watching her and had put a hex on her.

Judge Denise Sher earlier allowed Brewer to retain custody despite knowing the mom once told Demesyeux she would “throw the kids out of the window,” if he failed to show up for scheduled visitations and failed to submit to court-ordered drug tests, the dad said.

“They would have a conference with the judge [Sher] and the judge would say, ‘Let’s give her one more chance,’ ” the dad said.

County Executive Tom Suozzi – who cut short a family vacation in Florida to deal with the tragedy – ordered a full investigation.

“Obviously something went seriously wrong and we need to determine whether part of that wrong was with the system itself,” Suozzi said. “I’ve ordered an immediate review of anything related to this case in the Department of Social Services and the Police Department.”